This invention relates to vacuum gauges and, more particularly, to improvements to cold cathode ionisation vacuum gauges, especially those of the inverted magnetron type.
Cold cathode ionisation gauges for measuring vacuum, sometimes referred to as "Penning" gauges, are well known. They generally comprise an anode and one (or more) cathodes with a large potential difference between the anode and the cathode(s) and a substantial magnetic field applied by a permanent magnet in the area between the electrodes. In these gauges, the anode and cathode are held in a predetermined configuration relative to each other by means of a vacuum feedthrough which isolates the electrodes within the gauge from the atmosphere outside.
With the gauge subjected to the vacuum to be measured, electrons emitted by the cathode are accelerated towards the anode by the electric field; however, the action of the magnetic field causes the electrons to adopt a very long, non-linear trajectory, for example helical, before striking the anode; as such, the probability of ionisation of gas molecules present in the vacuum by collision with the electrons is much higher even at low pressures. Positive ions formed by the collisions are attracted by the cathode to produce an ion current in an external circuit, the size of which is related to the vacuum at a given temperature.
In gauges of the inverted magnetron type, the cathode commonly has associated with it means to define a region within it in which the ion collection, and hence ion discharge current to the external circuit takes place and is measured. This is generally achieved by arranging for the magnetic and electric fields to be concentrated in that region.
In such inverted magnetron gauges, it is possible for a small current to flow directly from the anode to the cathode via the internal surfaces of the gauge and it is known that the presence of a so-called "guard ring" can collect this current and thereby prevent it being detected by the gauge itself. To perform this function, the guard ring is electrically isolated from the cathode and normally held at a small positive potential relative to the cathode.